The present invention relates, in general, to the field of processing of signals received from a sensor array and, in particular, to a smart antenna system for separating and reconstructing a symbol stream generated by an individual user in a code division multiple access (CDMA) communication system.
Without limiting the scope of the invention, its background is described in connection with code division multiple access telecommunication devices and systems, as an example.
Heretofore, in this field, a user using a wireless handset transmits information from the handset to a base station. In commercial applications the wireless handset is most commonly a cellular phone or a personal communication system (PCS) phone. In military and other applications involving wireless transmission, the wireless handset may be generally be any mobile radio apparatus such as a spread spectrum transceiver or a mobile satellite terminal. A communication protocol used to transmit and receive wireless signals between the wireless handset and the base station is called an air interface. The air interface is typically agreed upon by an international standards committee.
Common air interface standards include analog service, time division multiple access (TDMA) and code division multiple access (CDMA). The most common CDMA air interface is called IS-95, based on the ITU-T IS-95 standard. CDMA technology and related spread spectrum coding techniques are discussed in detail in the ITU-T IS-95 standard, and in a variety of communications technology references. These references are useful in providing a systems level understanding of CDMA and emerging wideband-CDMA (W-CDMA) technologies.
Within CDMA and WCDMA applications, certain spread spectrum processing occurs at an inner spreading layer within the spread spectrum coding architecture. For example, a reverse channel (uplink) of one type of W-CDMA system includes a pair of in-phase(I) and quadrature(Q) outer-layer long codes and an inner layer orthogonal code. The orthogonal code has 64 chips per symbol. Depending on a particular system architecture, an inner-layer orthogonal code is typically 64 chips long and repeats every symbol. An outer layer long code and/or short code in a given CDMA system are de-spread, prior to de-spreading the orthogonal code. Hereafter, reference to a user""s spreading code indicates the user""s orthogonal code as found at an inner layer of a layered CDMA coding architecture.
A wireless communications antenna is commonly sectorized; each sector utilizing a portion of the operational frequency spectrum. Sectorization provides various functionalities to system operators and mobile units; frequency re-use amongst different sectors being a primary concern of system operators. Technologies such as CDMA allow xe2x80x9csharingxe2x80x9d of a sector""s frequency spectrum by multiple mobile units.
While CDMA is effective at increasing the capacity of wireless systems when compared to analog and TDMA (time division multiple access) technologies, there is increasing demand to further increase capacity. Increased capacity means more users can be serviced using with same amount of frequency spectrum resources in a given geographical area.
One method of increasing capacity is to use a smart antenna system. In general, a smart antenna system may rely on a time-varying beam pattern instead of sectorization. Conventional techniques generally use spatial filtering to separate communication signals by exploiting diversity in the spatial coordinates of their sources. Some methods, such as those based upon beamformer algorithms, make use of a signal propagation model which directly incorporates direction of arrival information. However, current beamformer approaches are limited in the number of beams which can be formed by an antenna array with a fixed number of antennas. Similarly, matrix based signal copy algorithms such as those derived from eigenstructure-oriented direction finding techniques are limited in the number of signals they can resolve for a fixed number of antennas, are sensitive to modeling errors, and have difficulty in effectively dealing with multi-path signals.
One class of conventional antenna system utilizes a front end spatial beam-steering processor coupled to a standard receiver, such as a xe2x80x9crakexe2x80x9d receiver (RAKE). Some systems of this class are generally categorized as switched beam systems. Switched beam systems (SBS) are similar to antennas having fixed and sectorized fields of view, but fields of view in a SBS are typically more directive and may be electronically xe2x80x9csteeredxe2x80x9d in response to system loading conditions. Other systems of this class are generally categorized as adaptive arrays.
Adaptive arrays form individual beams to isolate a signal due to a particular user. Conventional adaptive arrays generally perform adaptive spatial filtering to isolate a user""s signal, and then pass this spatially-filtered signal to a down-stream processor for de-spreading and equalization. Such a system may use an architecture which requires a bank of correlators for each multi-path of each user""s signal received from each antenna. Outputs of the correlator banks are processed by RAKE beam-formers. Such architecture makes modeling assumptions about the array pattern, which can lead to performance degradations due to modeling error sensitivity.
Recently, a diversity-reception antenna system has integrated CDMA interference suppression, noise suppression, and multipath interference suppression into a signal optimized structure. The system uses an orthogonalizing adaptive filtering approach which performs decision-directed MMSE (minimum mean squared error) updating. This system is applied with a diversity combiner which is used to add together outputs of various diversity paths to form a decision statistic. Individual paths are adapted either individually or collectively based upon the most reliable path, depending on the embodiment.
While such a system provides a diversity-reception device based upon orthogonalizing filters, characteristics of the approach are undesirable. Decision-directed adaptation often becomes unreliable and fails and under severe interference conditions. Also, these systems involve sub-optimal diversity combining and do not perform jointly optimized space-time processing. All adaptive optimization is performed in the time domain to orthogonalize user signals, but no joint-spatial adaptive processing is employed which also orthogonalizes a desired signal from interference in the spatial domain.
Such approaches can be improved upon using various block-adaptive algorithms together with fully blind cost functions. Block-adaptive algorithms may involve, for example, a block-gradient descent algorithm, a block conjugate-gradient algorithm, a block-Gauss-Newton algorithm, or a block-Shanno algorithm. In general, a block-nonlinear optimization algorithm may be used to cause a CDMA user signal to be demodulated, so as to minimize a nonlinear objective function, such as a constant-modulus error function. Thus, reduced bit errors rates may be achieved across a broad range of signal and interference scenarios with a lower computational complexity as compared with the previous approach.
Conventional antenna array signal processors, based on a constant modulus algorithm (CMA), typically involve a spatial-domain set of parameters which are adapted to cause a demodulated signal to have a constant modulus. Some algorithms of the CMA array type involve a space-time beam-former structure having both space domain and temporal domain taps. However, these methods generally serve as front end processors and do not provide orthogonalizing structures for isolating CDMA signals in both a chip-domain and a spatial-domain.
Another conventional approach involves xe2x80x9csubtractive CDMAxe2x80x9d. In subtractive CDMA, a user""s signal power is deduced. When a symbol decision is made based on a user""s signal having strong power, the symbol decision is re-spread, weighted, time-aligned, and subtracted from the composite CDMA waveform. This allows a weak signal to be recovered with higher reliability when received in the presence of strong interference from another user""s signal; providing a degree of near-far resistance by reducing the effect of a near (high dower) user""s signal on the reception of a far (low power) user""s signal. However, subtractive CDMA suffers in performance because at the receiver, a user""s spreading code has undergone channel distortion and/or synchronization-related distortion, and is thus not orthogonal to all other user""s signals. Hence, subtracting a very powerful user""s spreading code can reduce signal strength of a weaker signal, thereby causing a degradation in overall performance.
In a present subtractive CDMA system for use with smart antenna systems, a plurality of versions of a composite CDMA waveform are received from a plurality of antennas. A matrix containing these signals is stored and a two-dimensional transform is computed to separate individual users"" signals in time based upon their spreading codes, and in space based upon a set of beam patterns. This transform produces a two-dimensional matrix whose (i,j)th element corresponds to a demodulated output related to the ith user""s spreading code as viewed from the jth beam. The (i,j)th element of the transformed matrix with the largest norm is then used to form a decision for the ith user. Next, the (i,j)th element of the transformed matrix is set to zero (i.e., a subtraction), and the transformed matrix is then inverse transformed. This creates a new input matrix whose strongest signal component has already been decoded in both space and time and has been removed. Using this new input matrix, the process is repeated. While overcoming some limitations of other methods, this method is quite costly; requiring computation of the forward and reverse two-dimensional transforms for each user. Also, this method does not address the fact that the spreading codes of the users undergo distortion and therefore involve non-orthogonal components. Fixed beams are used in the antenna processing and therefore non-optimal spatial orthogonalization is achieved.
A need has therefore arisen for a smart antenna system which overcomes the aforementioned limitations of conventional systems. A need has arisen for a space-time processor architecture which is able to more selectively separate (orthogonalize) user signals using an approach which is not prone to modeling errors. A need has further arisen for an architecture which does not need to form explicitly one beam for each multi-path of each user, but is able to jointly and optimally separate user signals in space and time using a single orthogonalizing filter structure.
A need has also arisen for an architecture which is adapted to jointly orthogonalize a user CDMA waveform in both the chip-domain and the spatial-domain. A need has further arisen to have such an architecture without requiring expensive matrix operations such as singular-value decompositions or eigen decompositions.
Finally, a need has arisen for a subtractive CDMA system which could orthogonalize user components in both the chip-domain and the spatial domain, so as to reduce effects of having strong nonorthogonal components subtracted from and reducing the power of weak signals. A need has further arisen to provide such a system without increasing, and preferably decreasing, complexity.
The present invention solves these and other problems by providing a smart antenna system which can efficiently and jointly optimize sets of chip-domain and spatial-domain system parameters. The present invention provides a smart antenna apparatus that separates an individual user""s signal from multi-access interference (MAI) and thermal noise, compensates multi-path fading effects, and discriminates user signals according to their directions of arrival.
The present invention is a continuation-in-part of a parent application filed on Jan. 4, 1999, Serial No. 60/114,637. The parent application is entitled xe2x80x9cAdaptive Multiple Access Interference Suppressionxe2x80x9d, and is herein incorporated by reference.
In the present invention, a smart antenna system extracts a user""s data-symbol stream from a composite CDMA waveform. As used herein, a xe2x80x9ccomposite CDMA waveformxe2x80x9d is a signal at any layer of a CDMA coding hierarchy comprising a plurality of CDMA user signals. Various types of composite CDMA waveforms may be constructed according to the specific system modulation physical layer description. For example, some composite CDMA waveforms may involve additional types of modulations such as multi-carrier modulation. A space-chip processor structure according to the present invention provides low-cost means to orthogonalize a user""s signal from interference components jointly, in a both a chip domain and spatial domain, using a block-adaptive nonlinear optimization algorithm.
As defined herein, xe2x80x9cchip domainxe2x80x9d is a vector space comprising the set of orthogonal code vectors as typically found at an inner layer of a spread spectrum coding architecture (e.g. C64). xe2x80x9cSpatial domainxe2x80x9d is a vector space spanned by a set of vectors whose elements are derived from a plurality of antennas in a smart antenna system. In general, the spatial domain vectors must have elements from at least two different spatial sample points and may optionally include temporal elements as well. For example, each antenna may provide several output sample streams which have identical chip spacing but are staggered in time with respect to each other by a fractional chip.
The present invention provides a bilinear orthogonalizing filter system used to isolate an individual user""s signal from a composite CDMA waveform. A bilinear orthogonalizing filter structure jointly orthogonalizes a user""s signal from other user""s signals in both the chip domain and the spatial domain.
Further, the present invention provides a system of smart antenna digital signal processing for CDMA signal detection. This system exploits additional degrees of freedom, increasing the range of a base station and the number of users which can be serviced under a fixed set of power and frequency resources. Improved subtractive CDMA multiuser detectors are also developed for single-antenna and smart-antenna systems.
In a preferred embodiment, this filter structure comprises a bilinear transformation. Preferably, a block-adaptive nonlinear optimization approach is used derive a set of chip domain parameters and a set of spatial domain parameters used in the bilinear orthogonalization. The present invention thereby provides an effective means to extract an individual user""s signal from a composite CDMA waveform.